The present invention relates to devices capable of precisely positioning materials on a substantially planar coordinate platen. More particularly, the invention relates to means for precisely superimposing photographic masks on a photosensitive film or lithographic printing plate which is retained within the frame of a plate making machine.
In the photolithographic process, it is frequently desirable to have multiple images in spaced and predetermined positions on the photographic plate, which images are obtained by repeated exposure. Certain economies result from the production of a number of identical reproductions from a single plate impression, and in color work, the number of impressions is equal to the number of colors required. In order to reproduce work of highly acceptable quality, it is necessary that the various color impressions be exactly superimposed. To this end, it is necessary that the negatives, positive flats or photographic masks use to expose the photolithographic plate be in precise register and proper contact. While devices of this general character are known, because of the extremely accurate machine work required in the fabrication thereof, and the large bulk and weight involved, such devices are relatively costly, and the use thereof is consequently restricted. In addition, the systems employed in prior art machines are complicated and require considerable training before the operator is capable of reproducing satisfactory work.
U.S. Pat. Nos. 3,150,582; 3,484,166 and 3,639,056 which are incorporated herein by reference, describe devices which support a negative carrier in such manner that it may be moved in a plane over the face of a support for the lithographic film or plate, and be locked in proper location in any one of a plurality of positions whereby successive exposures may be made.
While the above-mentioned means are extremely useful, nevertheless, where a plurality of lithographic films or plates are exposed, it is still necessary to precisely align the negative carrier each time the negative carrier is moved to a new location. The locating of the negative carrier must be determined by reference to a scale, first with respect to a horizontal position, and secondly with respect to a vertical position. This has been time-consuming.
For example, as in U.S. Pat. No. 3,639,056, a negative carrier is moved along a plate carrying frame by means of a toothed rack along a part of the perimeter of the frame. The position of the carrier is marked by means of a series of electrical contact clips or keys mounted on a second rack parallel with the rack bearing the negative carrier. When the carrier is properly aligned an electrical connection is made between the carrier and the contact clip so as to illuminate an indicator light.
This method is disadvantageous for several reasons. First, the contact clips are costly to manufacture. Second, the clips are usually repositioned for each job and much machine and operator time is wasted during this set-up operation. The clips must be individually repositioned.
The present invention improves upon this arrangement by replacing the clip carrying racks with either transparent strips having opaque indicia or opaque strips having punched holes, at the sites of horizontal or vertical positioning. The strips are positioned in the groove of an optical interruptor fixed to the negative carrier transport mechanism. When one of the indicia is positioned in the groove, the interruptor activates an indicating circuit. The strips are quickly interchangeable and may be prepared while they are off the plate making machine.